Practical photographic emulsions are microcrystals of silver halide which are treated with certain chemicals to enhance their sensitivity to light. The photographic industry is engaged in continual efforts to modify the processes of making and sensitizing silver halide grains to obtain improved speed/fog relationships.
The manner in which iodide is introduced into silver halide grains has been shown to be important. The importance of the spatial relationship between halide phase boundaries and chemical treatment has also been described. It has also been recognized that photographic sensitivity can be increased by adjusting the pH and/or the pAg of the silver halide emulsion. It is also known that photographic speed can be enhanced by the addition of emulsion adjuvants such as alkyl ethers of polyethylene glycols; poly(thiodiethylene glutarate); azaindenes and mercaptoazoles. Certain reducing agents such as stannous chloride, ascorbic acid, dimethylamineborane have also been described as being useful to increase sensitivity. U.S. Pat. No. 3,893,862 describes the use 1,4- and 1,2-dihydropyridine compounds to increase effective speed upon development.
One problem which has been encountered in the development of highly sensitized emulsions is that of pressure desensitization. For example, emulsions containing grains exceeding 1 micron, particularly those having a tabular morphology and a halide phase structure designed for optimum light sensitivity, are strongly desensitized by the application of mechanical pressure to a coated film. Emulsions which have been sensitized by the introduction of localized regions of very high iodide content are particularly sensitive to mechanical pressure. Although emulsions can be designed which are highly resistant to this pressure induced desensitization, their light sensitivity is often inadequate to construct films with acceptable granularity.
Even with all the effort to develop highly sensitive emulsions, there is still a need for improved sensitizing agents, especially those which can be utilized over a wide range of pH and pAg without deleterious effects.
There is also a continuing effort in the industry to provide methods of modifying crystal habit to obtain crystal surfaces which differ from the conventional cubic and octahedral crystal faces. The reason for this is that the light absorption and stability of silver halide crystals is affected not only by the crystals' shape, but also by the different silver and halide ion distributions on the crystal faces which result in different surface electric fields. Further, photographically active materials such as dyes and chemical sensitizers absorb differently to different crystal faces thereby leading to more or less efficient latent-image formation.
It is thus a desire in the industry to provide new and alternative methods for modifying crystal growth. This will provide an opportunity to obtain silver halide crystals exhibiting advantageous characteristics such as high sensitivity and stability.